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This I like, thank you for taking the time to think about it.An alternative, depending on the cable runs involved, would be to put three DP contactors next to the DB, one per heater, each fed from an SP MCB and feeding the heater connection unit. Fit one stat in each room wired back in 1.0mm² to the contactor box. Feed the timer from a 6A control MCB either in the board or the contactor box, then take its output to both stats and the returns from the stat to the appropriate contactors. Advantages include not switching the heater load with the stats so they should last longer, and simplicity; three identical single-phase power circuits and one single-phase control circuit.
I agree yours is a more elegant system, but I wanted to avoid the complication and possible future confusion of feeding stat control lines back to the DB area. The runs are not easy.
I certainly agree about causing the stats to last longer - I like using relays for all kinds of things, and personally never use 13A stats direct wired on anything over about 6A, which this is, just!
I was using a 3P MCB only because there's a 4-pole contactor, and I like reducing component count. The 10A 3P MCB would simply be labelled 'Heating'.
Bloody hell this is a difficult call and I need to order parts and finish it by Thursday.
Here's a compromise I alluded to earlier: Keep the 3P MCB and one timer-circuit-controlled 4-pole contactor, which can be inside the DB so no external box is needed. Feed to three quality 13A stats, direct wired to the 6A heaters.
Do you think that under-run electronic stats with internal encapsulated relays are any less reliable than contactors? I agree that the old bi-metal type are trash, run hot and burn out, but those days are over. Aren't they?